1. Field of the Invention
This invention relates to a heating method of a semiconductor wafer, and more particularly to a heating method of a semiconductor wafer which may have a first region to be heated and a second region requiring no heat thereof and overheating thereof should be avoided.
2. Description of the Prior Art
A semiconductor wafer (hereinafter may be referred to merely as "wafer" for brevity) is used as a substrate for fabricating a semiconductor device such as an integrated circuit or large-scale integrated circuit. In a course of fabrication of such a semiconductor device, a variety of heating steps is required depending on what end use would be made on the semiconductor device. Among such heating steps, there are for example an annealing step for healing crystal defects in an ion-implanted layer of the wafer, a thermal diffusion step for thermally diffusing dopants incorporated in the wafer, a heat treatment step for activating dopants, etc. As a method for conducting, for instance, the annealing step out of the above-mentioned various heating steps, there has conventionally been known to heat a wafer in an electric resistive furnace. Reflecting the recent demand for higher densification of semiconductor devices, it is now required to control a pattern of distribution of dopant atoms along a surface of the wafer more minutely. Thus, it is no longer permissible to ignore thermal diffusion and redistribution of dopant atoms along the surface of the wafer which take place upon annealing each wafer. Owing to the above problem, it is now required to make the annealing time as short as feasible. However, it is difficult to conduct a sufficient heat treatment of a wafer in the electric resistive furnace in a short period of time during which no thermal diffusion of dopant atoms or the like substantially takes place.
With a view toward overcoming the difficulty which electric resistive furnace have encountered, there has been developed a novel annealing method which makes use of laser beam or electron beam. This novel method is certainly effective in carrying out a heat treatment in a short period of time. However, it is accompanied by such problems that damages may occur in a surface of a wafer as the radiant beam is monochromatic having single wavelength and accordingly considerable interference of the radiant beam and reflected beam takes place and a problem of discontinuity or non-uniformity is developed along a boundary of each two adjacent scanning lines when a wafer is scanned by a single beam. Due to such problems, the above annealing method is not suited, especially, for annealing a wafer of large surface area.
With the foregoing in view, it has currently been attempted to develop a method for heating a wafer for annealing by exposing the wafer to a flash of light emitted from flash discharge lamps. Exposing to a flash of light permits to raise the temperature of the wafer to a desired level in a short period of time during which no undesirable problems takes place. In addition, a flash of light, in other words, flashlight is not light of a single wavelength and is thus less susceptible of developing interference of the light, thereby successfully avoiding development of damages in the surface of a wafer. Furthermore, flashlight is not a beam and, corollary to this, does not require to scan the wafer. Therefore, heating process by exposing to flashlight is free of the problem of discontinuity or non-uniformity which is developed along a boundary of each two adjacent scanning lines when the wafer is scanned. Thus, application of a flash of light for annealing a wafer has another merit that the wafer may be of a large surface area.
It is rather rare that a wafer to be subjected to a heat treatment has a uniform reflectivity all over the surface thereof. Generally, a variety of layers such as, for example, an ion-implanted layer, a mask layer made of an oxide film for the ion implantation and the like is formed in a surface of a wafer which is to be heated for its annealing. In a wafer, there are thus a portion which requires heat treatment and a portion which does not require such a heat treatment and should not be overheated, and the former portion (hereinafter called "the first portion") and the latter portion (hereinafter called "the second portion") are generally different in reflectivity. Due to the difference in reflectivity, the final temperature of the first portion is different from that of the second portion no matter how precisely the radiation source, namely, the radiation intensity of each flash of light is controlled. As a result, there is such a problem that the first portion may not always be heated to a desired temperature level and the second portion may, instead, be exposed to undesirable elevated temperature higher than that of the first portion and hence damaged.